The present invention relates to apparatus and methods for treating solid substrates with a gas.
One gas treatment process which is widely utilized in production of semiconductor devices is the gas deposition process. In this process, a substrate such as a planar wafer of silicon or other suitable material is exposed to gases which react at the substrate to deposit the desired materials on the front face of the wafer. Typically, the deposited materials form epitaxial films which replicate the crystal lattice structure of the underlying substrate. Several different reactive gas mixtures may be employed in succession to deposit layers of differing composition. Thus, a mixture of hydrogen, silicon halides and halides of a desired minor ingredient or dopant may be contacted with the wafers while heating the wafers. Upon contact with the heated wafer surface, the gases react to deposit a layer of silicon containing the desired dopant on the wafer surface. The process is repeated using different dopants to provide a multilayer semiconductor structure including several layers having different dopants. Similar processes may be employed with mixtures of trimethyl gallium and arsine to deposit layers of gallium aresenide, likewise with desired dopants or minor ingredients.
The coated wafers are subjected to well known further processes to form devices such as integrated circuits. The layers deposited on the wafer in the gas deposition process form the active elements of microscopic transistors and other semiconductor devices included in the integrated circuits. The thickness, composition and quality of the deposited layers determine the characteristics of the resulting semiconductor devices. Accordingly, the gas deposition process must be capable of depositing films of uniform composition and thickness on the front face of each substrate. The requirements for uniformity have become progressively more stringent with the use of larger wafers and with the continuing reduction in the size of the semiconductor devices fabricated from the coated wafer.
It has been difficult heretofore to maintain a large number of substrates at a uniform temperature. Typically, the substrates have been heated by contact with electrically conductive supports or susceptors which in turn have been heated by electromagnetic fields. Nonuniformities in the electromagnetic fields have typically resulted in non-uniform heating of the susceptors and wafers, resulting in non-uniform reaction of the gas at the substrates, and hence in non-unifrom deposition. Moreover, nonuniformities in flow of the depositing gas over the wafer surfaces heretofore have resulted in nonuniform contact between the gas and substrate and hence in nonuniform deposition. These difficulties become more severe when a plurality of wafers are processed simultaneously. Further, the apparatus utilized heretofore typically has not been well suited to rapid loading and unloading. Thus, considerable time is consumed in placing the substrates or wafers within the chamber and removing then from the chamber.
Various attempts have been made to overcome some or all of these difficulties. As set forth in U.S. Pat. No. 3,460,510, a plurality of wafers may be mounted on either the exterior or interior surface of a cylindrical support disposed within a chamber, and the support may be rotated to provide a more uniform heat distribution. Likewise, U.S. Pat. No. 3,384,049 discloses vapor deposition apparatus with ring-like susceptors or supports, which are rotated about a vertical axis. The wafers or substrates are disposed in pockets on the interior surface of these ring-like susceptors, so that so-called centrifugal force retains the wafers in the pockets. The depositing gas is introduced into the chamber through a perforated feed pipe extending into the chamber along the vertical axis of rotation of the ring-like susceptors, so that the gas passes outwardly from the feed pipe and contacts the inwardly facing wafer surfaces. U.S. Pat. No. 3,659,552 teaches essentially the same approach. Further, U.S. Pat. Nos. 3,407,783 and 3,408,982, disclose vapor deposition apparatus in which the wafers or substrates are disposed on a rotatable disclike support, and the depositing gas is discharged through the center of the disclike support so that the gas flows radially outwardly over the faces of the wafers. The article "Characterization of GaAs Films Grown by Metal Organic Chemical Vapor Deposition," by Swaminathan et al, Journal of Applied Physics, Vol. 57, No. 12, pp. 5349 et seq., (1985) reports deposition of gallium arsenide films on substrates rotated at rotational speeds of between 50 and 1,000 revolutions per minute on a rotary pedestal within a bell jar reactor.
Although the methods and apparatus set forth in the aforementioned references are said to ameliorate the difficulties in the gas deposition process, there have still been needs heretofore for further improvement.
Another gas treatment process applied to semiconductor wafers heretofore is gas etching. In the gas etching process, a wafer having a pattern of masking material on its surface is exposed to a gas, typically an ionized gas, also referred to as a plasma. The ions in the plasma attack those regions of the surface not covered by the masking material, thereby forming discrete microscopic features defining the semiconductor devices of integrated circuits. Difficulties similar to those referred to above in connection with the gas deposition process have also occurred heretofore in the gas etching process. Thus, nonuniform contact between the plasma and wafers, or nonuniform temperatures can lead to undesirable nonuniform etching rates. There have accordingly been corresponding needs for improvement in gas etching processes, and in other processes wherein a gas is employed to treat a solid substrate such as a semiconductor wafer.